When California residents voted to permit the use of recreational marijuana last November, the state became one of eight U.S. jurisdictions (along with Washington, Oregon, Nevada, Colorado, Alaska, Hawaii, Maine, Massachusetts and the District of Columbia) to permit the practice. An additional 20 states have laws allowing marijuana use for medicinal purposes.
While the new marijuana laws may offer some relief to people suffering from painful illnesses—and provide a popular alternative to alcohol for those who want to party or simply relax— they are posing a real dilemma for law enforcement officials charged with keeping DUI drivers off the road. The problem? At present there is no widely accepted test or measurement that defines whether a person is too intoxicated by marijuana to drive safely.
Major differences between marijuana and alcohol
Colorado, Washington and Montana have all adopted a DUI standard that allows prosecution of drivers who have five nanograms per milliliter (5 ng/mL) of marijuana’s active ingredient, THC (tetrahydrocannabinol), in their blood. But many people take issue with this arbitrary limit, and marijuana proponents aren’t the only ones disputing its accuracy. The AAA Foundation for Traffic Safety, which conducts numerous studies on many topics related to driving safety, states clearly on its website that “legal limits, also known as per se limits, for marijuana and driving are arbitrary and unsupported by science.”
Why can scientists feel so confident that the 0.04 DUI limits for alcohol is correct and be so unsure about the same type of limits for cannabis? Much of it has to do with the way that the body metabolizes the two substances.
In its March 2016 publication, “Overview of Major Issues Regarding the Impacts of Alcohol and Marijuana on Driving,” the AAA Foundation spells out some of the big differences between alcohol and marijuana.
1. Producers of alcohol products can (and must) clearly define the volume of alcohol contained in their offerings. That’s much harder to do with marijuana, since THC, the element that typically causes the cannabis high, exists in varied concentrations in different plants and even in different parts and/or extracts drawn from the same plant. Plus, many states do not require marijuana producers to list the amount of THC in their products.
2. Alcohol moves readily throughout the body, so breath or blood levels reflect its concentration in the central nervous system. THC, on the other hand, concentrates in fatty tissue and is less evenly distributed through the body than alcohol is. That means that blood level readings may not truly reflect the effects of THC on the central nervous system.
3. Blood alcohol concentrations (BAC) correlate fairly closely to levels of impairment, i.e., if someone’s BAC measures higher than .04 percent it’s likely that person is too impaired to drive. In contrast, the levels of THC in the bloodstream decrease at a faster rate than alcohol, but the THC levels don’t correspond to the levels of impairment. People who smoke marijuana remain impaired even two to four hours after the concentration of THC in their bloodstream drops to a low level. If someone eats a food that contains marijuana (like brownies or cookies), the THC level may never be as high as it would be if they smoked weed but they would still be substantially impaired.
4. Because of the way that the body metabolizes alcohol, the breathalyzer is a fairly accurate measurement of a person’s BAC. That’s not the case with THC; the most accurate and reliable way to measure its presence in the bloodstream is with a blood sample. Since THC disappears from the bloodstream quickly, an accurate reading requires collection of a blood sample within an hour of driving.
5. Alcohol leaves the bloodstream at a fairly constant rate, so law enforcement officials can make a good estimate of a suspected DUI driver’s BAC up to two hours before the blood test is performed. THC, by contrast, can remain in lipid membranes and may be detected by a blood test for weeks after someone smokes or ingests marijuana. There’s no good way to estimate what a THC level had been an hour or two before law enforcement measure it.
6. Alcohol usually affects drivers’ psychomotor functions, their ability to detect signals, perceive hazards and react quickly. Alcohol is more likely to impact tasks that require conscious effort, rather than the more automatic tasks. That’s just the opposite of what happens with THC; it is more likely to affect automated or routine driving tasks, but it may also impact coordination, attention and vigilance while driving.
The AAA Foundation for Traffic Safety sponsored a study that evaluated whether or not data from drivers arrested for suspected marijuana DUI supported any per se law for THC. The researchers looked at the data from more than 5,300 drivers arrested for DUI where THC was present and compared them to drug-free control subjects. The study found:
• People with THC in their bloodstream showed significantly more incidence of red, bloodshot and watery eyes, lack of convergence (ability to cross eyes to look at an object moving closer) than THC-free subjects.
• In most impairment tests, people with a measurement of 5 ng/mL of THG or higher in in their bloodstreams showed no difference than people who had less than 5 ng/mL THG. The only difference was in the finger-to-nose test.
• The researchers weren’t able to identify a threshold THG level (in a range of 1-10 ng/mL) that correlated with the results of the Standardized Field Sobriety Tests (SFSTs).
The researchers concluded that “All of the candidate THC concentration thresholds examined would have misclassified a substantial number of drivers as impaired who did not demonstrate impairment on the SFST, and would have misclassified a substantial number of drivers as unimpaired who did demonstrate impairment on the SFST. Based on this analysis, a quantitative threshold for per se laws for THC following cannabis use cannot be scientifically supported.”
As more and more states legalize marijuana for medical and/or recreational use, the number of impaired driving incidents connected to the drug is likely to increase. The AAA Foundation contends that “Fatal crashes involving drivers who recently used marijuana doubled in Washington after the state legalized the drug…These findings serve as an eye-opening case study for what other states may experience with road safety after legalizing the drug.”
Scientists are continuing their attempts to find more accurate ways of measuring marijuana DUI impairment.
Engineers at Stanford University have developed what they claim could be the first practical roadside test for marijuana intoxication. The test relies on magnetic nanotechnology, which is already used to do cancer screening.
A team of researchers led by Shan Wang created the “potalyzer,” which they say can quickly detect not only the presence of THC in a person’s saliva but also measure its concentration. After collecting a spit sample, law enforcement officials could analyze it using an app installed on a smartphone or laptop.
The researchers have focused on saliva samples because they say that the THC in saliva may give a better indication of driving impairment than THC in either blood or in urine.
At the University of Massachusetts in Boston, psychology professor Michael Milburn has developed an app (named DRUID for Driving Under the Influence of Drugs) that he claims will help law enforcement officials determine whether or not a driver is DUI after using marijuana. According to the Boston Globe, the app gives people a series of tests, including tapping a screen on an electronic device when they see different shapes and following a moving circle on the screen with a finger. Milburn, who plans to sell the app for 99 cents, believes that it could be as accurate as a breathalyzer test for alcohol.